344 TE VOR 
In Nos. 24, 25c% and c?, 26, and 27, where plagioclase (or perhaps 
predominant plagioclase and some orthoclase) crystallized at an 
early stage, respectively: 0.40, 0.38, 0.41, 0.38, and 0.36 Or. 
The remainder is Ab+an. 
These values, calculated from the analyses, for the proportions Or: Ab+An 
need, however, a small correction, as we have taken for granted that the whole 
quantity K.0, Na.O, and CaO form respectively Or, Ab, and An, while in 
reality a trifle K,O (and a still smaller amount of Na.O) in several cases enters 
into biotite. 
3. The glass basis, or the groundmass, in andesites (with at 
least about 56 per cent SiO., that is to say, with at least so much 
SiO, that a little of the independent component quartz entered 
into the melted magma) and also in dacites, trachytes, rhyolites, 
quartz-porphyries, etc. (with max. about 72 per cent SiO.) without 
exception shows an increased percentage of SiO, as compared with 
the entire rock. Gradually as the crystallization, for example, of 
an andesite with s9 per cent SiO, (No. 14a) advances with solidi- 
fication of magnetite, ferromagnesian silicates, and medium-basic 
plagioclase, the temperature drops, and simultaneously the mother 
liquor becomes richer in SiO,. By rapid cooling the viscosity 
increases and causes the cessation of the crystallization; in other 
words, the fluid remnant stiffens into glass, with varying percent- 
ages of SiO, according to the time at which the crystallization 
ceased. And this point of time may lie even considerably lower 
than the stage of the final eutectic. This we may illustrate by 
giving the percentages of SiO, in the entire rock and in the glass 
basis (or in some cases the groundmasses); the latter, however. 
always contains a little H,O, showing that it is somewhat decom- 
posed. (See my treatise in Tscherm. Mitt., Vol. XXIV [r1905].) 
ANDESITES 
Percentage of SiOz in 
ROC a ielt Lacnss okane 56.8 §7.8 58.1. 50.1 60.1 60.4) (6270 G2) suycemm 
Glass basis....... 64.5 65.1 70.8 68.1 68:7 68.6 60.0)9607 ON jome 
DACITES 
Percentage of SiO: in 
ROCK oc Ae eoheosateet ere O56) 10S 43) 00h4) s O5mS O5.5 67.3 
Glass. basis )?\ae% 302 sees 41.9 74.8% 76.75 70.2 70.2 ald) 73-0) age 
* Goundmass. 
